This invention relates to safety system for a materials handling device.
Conventionally, where materials handling devices are to be moved around industrial premises, such as warehouses or factories, it is to be expected, in certain industries, that the device may be required to move through areas in which hazardous substances are present, e.g. explosive vapors. Legislation defines a level at which explosive vapors are present known as a lower explosive level (LEL). Areas in which the level of explosive vapors is less than 25% LEL are generally regarded as safe areas, whereas areas in which the level of explosive vapors is greater than 25% LEL are regarded as hazardous areas. Legislation further defines the level of protection which a materials handling device is expected to have to be able to move around in areas in which the level of explosive vapors is greater than and less than 25% LEL.
In order to comply with the legislation, it is known for materials handling devices such as fork lift trucks, conveyor belts, etc. to have safety systems which are designed to ensure that hazardous; components of the device, such as electronic circuitry which might produce an occasional spark or arc, are isolated from the explosive vapors in areas where such vapors are detected. Generally speaking, these conventional safety systems enclose hazardous components in sealed enclosures which are filled with an inert gas at a pressure greater than atmospheric pressure to prevent explosive vapors reaching the hazardous components of the device. If the pressure is maintained above atmospheric pressure, the device is suitable for use in areas in which the level of explosive vapors are both below or above 25% LEL.
However, these conventional arrangements suffer from the problem that whilst allowing the device to move between areas in which the level of explosive vapors is less than or above 25% LEL they have a relatively short operational time. This arises due to the fact that the devices normally carry on board supplies of inert gas and, if the pressure of inert gas is kept above atmospheric pressure to render the device capable of being used in above 25% LEL conditions, the supply of inert gas is exhausted within a short space of time.
There has been a long felt need for a materials handling device which can move in above and below 25% LEL conditions and which has a reasonably long operational time.